1. Field of the Invention
The present invention relates generally to the installation of sanitary sewers and more specifically, to the problem of tightly sealing a manhole against inflow or outflow of liquid at the point where a sewer pipe passes through its walls and still more specifically to elastomeric gaskets for resiliently supporting and sealing the space between a pipe and the surrounding surface of an opening in the manhole through which the pipe passes.
2. Description of the Related Art
Pipes are commonly used for the conveyance of fluids in a variety of municipal applications, including their use as free-flowing conduits running partly full, as in drains and sewers. Pipes used in these type applications have been made of steel, cast iron, concrete, vitrified clay, and most recently, plastic including the various polyolefins and PVC.
In sewer lines, manholes are ordinarily installed at various intervals including locations where the sewer line changes elevation or direction. Manholes are typically formed in molds from a settable material such as concrete. The mold may form one or more annular openings in the manhole in which sewer pipes are positioned to lie adjacent a wall of the manhole defining that particular opening. The joint between the manhole wall and outer surface of the sewer pipe is sealed to prevent ground water from entering the manhole and to prevent contents of the manhole, such as sewage and water, from leaking out of the manhole. When ground water enters the manhole, it often brings in sediment which collects in the sewer over time. Such sediment causes clogs and inhibits the ability of the manhole to carry water. In addition, infiltration of ground water also increases the total volume of sewage flow through sewer lines which increases the required size of pipe needed as well as the expense of treatment of the total sewage flow. When the contents of the manhole leak, they can pollute the ground water.
Various types of gaskets for sealing joints between pipes and manholes are known. Often these gaskets are formed from an elastomeric material that is either inserted in an opening in the manhole and expanded via a ring or physically embedded in a portion of a wall adjacent the opening in the manhole. It is generally necessary to either physically embed the gasket in the concrete wall of the manhole or to use some type of physical expansion mechanism in order to insure that the gasket is properly retained in the desired position as the mating male pipe end is inserted into the manhole opening.
Expanded gaskets require rings that exert an outwardly, radially directed force on an inner surface of a gasket positioned within the opening to compress the gasket against a manhole wall defining the opening. These rings increase both material and labor installation costs associated with sealing the joint between the manhole wall and pipe. Material costs are increased because these rings are somewhat complicated corrosion-resistant metal structures that must be manually fabricated or are molded plastic structures that are made, at least in part, from high strength, expensive compositions. Labor costs are increased because installation and expansion of such rings to effect a seal requires maintaining proper alignment of the ring and use of equipment such as mechanical expansion units. In addition, seal failure will occur if the rings are improperly installed.
Embedded gaskets solve many of the problems associated with the manufacture and installation of expansion gaskets. Embedded gaskets also allow a larger diameter of pipe to be inserted into an opening in a manhole than expanded gaskets because a separate ring is unnecessary. As is known, outside pipe diameter size can increase up to a certain point at which an expanded gasket can no longer be used for a particular sized manhole because the width of a wall defining an opening in the manhole will no longer be large enough to support the expansion ring. This means that a next, larger sized, more expensive manhole must be used for the larger diameter pipe. However, embedded gaskets must be set in the wall of the manhole defining the opening for the mating male pipe. One of the problems with embedded gaskets is that they can, on occasion, roll and shift as the pipe is being inserted. It is critical, during the installation process, that the gasket not be able to twist or flip since a displaced or dislocated gasket will adversely affect the ultimate sealing capacity of the joint. Irrespective of the type of sealing action of the sealing element, such as compression sealing action, lip sealing action or a combination thereof, it is necessary that the sealing gasket consists of a relatively soft elastomeric material. However, a sealing ring which is formed entirely of a material which is sufficiently soft and elastomeric to provide the sealing function has the disadvantage that it is difficult to retain in the desired position. There is also the risk that such a sealing ring will be displaced from its sealing position if the difference of the internal or external pressures on either side of the sealing ring are sufficiently great. In addition, the rolling and shifting of the embedded gasket can weaken the actual manhole material itself and in turn adversely affect the ultimate sealing capacity of the joint.
What is needed is a gasket design which achieves the advantages of the embedded gasket but which will not tend to roll and shift as the pipe is being inserted within a manhole opening or be displaced from its sealing position due to a difference of internal or external pressure.
Also needed is such an improved gasket design which achieves these performance results without the need for external expansion rings or separate components to achieve ring expansion.